This invention relates to exhaust nozzles for directing gas turbine exhaust gas into the atmosphere to propel an airplane or other vehicle. More particularly, this invention relates to a variable centerbody plug-type exhaust nozzle and translating shroud assembly for providing optimum thrust over a wide range of flight conditions, including operation of an aircraft at subsonic, transonic, and supersonic speeds.
It is known that maximum thrust and operating efficiency of a gas turbine engine that supplies propulsive thrust is obtained when the engine exhaust effluent is directed through an exhaust nozzle that controls the expansion of the exhaust gases. Controlled expansion of the high temperature, high pressure gases supplied by the gas turbine engine increases the particle velocity of the exhaust effluent and thereby increases the momentum of the thrust exhaust producing stream. In this respect, maximum operating efficiency is generally achieved when the nozzle is configured to exit the exhaust stream at substantially the same pressure as that of the surrounding atmosphere.
When an aircraft operates at subsonic, transonic, and supersonic speeds, the exhaust nozzle pressure ratio, i.e., the ratio of the total fluid pressure upstream of the nozzle to the ambient atmospheric pressure, varies over a substantial range. In particular, under subsonic flight conditions, the nozzle pressure ratio is sufficiently low that full expansion is not required, while under supersonic flight conditions, the nozzle pressure ratio is quite high and proper expansion of the exhaust effluent must be effected. Moreover, fairly substantial variations in pressure ratio results from various engine throttle settings, and in some cases, also results from "ram effect" when an increased amount of air is effectively forced through the engine as the aircraft moves through the atmosphere at high speeds.
One way of achieving good performance under the various flight modes is by using an exhaust nozzle having a variable throat area to allow the expansion ratio of the exhaust nozzle to vary as the pressure ratio varies, thereby maximizing engine performance. As known to those skilled in the art, the expansion ratio of an exhaust nozzle is the ratio of the final area of exhaust gas when the exhaust gas is at ambient atmospheric pressure to the area of the throat or smallest cross-sectional flow area in the exhaust nozzle. Accordingly, many attempts have been made to design variable geometry exhaust nozzles that are operable to vary the throat area of the exhaust nozzle and the final area of exhaust gas exiting the nozzle. Variation of the throat area can be achieved by changing the geometry of the inner, central center portion of an exhaust nozzle, while the final area of exhaust gas exiting the exhaust nozzle can be adjusted by varying the geometry of the outer housing of the exhaust nozzle.
Although various nozzle configurations have been proposed to accommodate the requirement for both a variable throat area and the capability of varying the final area of exhaust gas, such prior art nozzles have not simultaneously met all of the necessary design criteria. Nozzles such as convergent-divergent nozzles and variable cross section plug-type nozzles have been proposed. The fixed geometry convergent-divergent nozzle performs well at design conditions, but has a drawback of severe thrust losses at less than the design pressure ratio. On the other hand, variable cross section plug-type nozzles, such as the multiple-leaf plug-type nozzle, have been proposed due to their good flight performance and favorable reduced jet noise. However, a major problem with prior art multiple-leaf plug-type nozzles has been the mechanical complexity involved with variation of throat area. Additionally, while the multiple-leaf plug-type nozzle provides very good area control, the leaves tend to leak when the centerbody is pressurized internally with cooling air. Making the leaves stiff enough so that they will seal under load imposes too large of a weight penalty.
Another factor to be considered in the design of an exhaust nozzle for use on a supersonic aircraft is that some supersonic gas turbine engine applications restrict the amount of engine shroud perimeter available for placement of thrust reverser cascades. In such a situation, reverser cascade length must generally be increased to maintain adequate thrust reverser flow area. Such an increase in length is usually accompanied by an undesirable increase in weight.
Accordingly, it is an object of this invention to provide a variable centerbody plug-type exhaust nozzle and translating shroud assembly for use on a gas turbine engine, such exhaust nozzle being operable over the normal flight regime of a supersonic airplane.
It is another object of this invention to provide a variable geometry exhaust nozzle of the above-described type wherein the geometry of the rearwardly extending centerbody and the position of the translating shroud can be continuously varied, either independently or simultaneously, to provide a wide range of nozzle throat areas and final expansion areas.
It is still another object of this invention to provide a variable geometry exhaust nozzle of the above-described type wherein adequate thrust reverser flow area can be maintained without increasing the exhaust nozzle length, and while using a limited amount of engine shroud perimeter for targeting the exhaust gas to preferred locations.
It is yet another object of this invention to provide an exhaust nozzle of the above-described type that is relatively light in weight, containable within a region of relatively low volume, and has a relatively uncomplex structure to reduce weight, facilitate manufacture, and enhance reliability.